1. Field of the Invention
The present invention relates to a multi-level information reproducing method, a multi-level information recording medium, a multi-level information waveform equalizing device, a multi-level information reproducing apparatus and a multi-level information recording apparatus.
Further, the present invention relates to a signal processing method, a reproduced signal processing circuit and an optical disk apparatus, and, in further detail, to a signal processing method and a reproduced signal processing circuit for processing a reproduced signal from an optical disk and an optical disk apparatus including the reproduced signal processing circuit.
2. Description of the Related Art
For example, Japanese Laid-open Patent Application No. 3-178038 discloses a reproducing method in which, when waveform shaping is carried out by means of a waveform equalizer circuit on a reproduced signal obtained from a recording medium, an equalizing coefficient of each tap is controlled according to an error with respect to a predetermined reference determination level, frequency characteristics are thus corrected, and thereby, not with the use of fixed waveform equalizing characteristics but the characteristics are corrected to optimum one based on the reproduced signal.
Further, Japanese Laid-open Patent Application No. 2002-319138 discloses a reproducing method in which, in order to correct characteristics of a waveform equalizer circuit in multi-level information recording, a pattern in which a record mark according to multi-level information is sandwiched by zeros, or identical record marks continue, is applied.
Recently, along with development of digital technology and improvement of data compression technology, optical disks such as a CD (compact disk), a DVD (digital versatile disk) in which data having a size approximately seven times that of a CD can be recorded in a disk having the same diameter as that of the CD, and so forth, take attention as information recording media to record information such as music, movie, photograph, computer software or such (referred to as contents, hereinafter). Accordingly, also along with reduction of the costs thereof, an optical disk apparatus handling such an optical disk as an information recording medium, has been widely spreading.
In this optical disk apparatus, a light source emits laser light, a microscopic spot is produced on a recording surface of an optical disk on which spiral or concentric tracks are produced, information is recorded thereon, and also, information thus recorded is reproduced therefrom based on reflected light obtained from the recording surface.
In the optical disk, in one way, information is recorded with the use of various lengths and combinations of marks and spaces which thus have difference reflectance. In this case, original information is first converted into combinations of two types of numerical values, i.e., 0 and 1, that is, the information is binarized. After that, the thus-binarized information is written in the optical disk. Such a recording method is referred to as a binarized recording method, hereinafter.
An information size of the contents to handle tends to increase year after year, and, then, a further increase in the information size which one optical disk can store therein is expected. In order to increase the information amount recordable by one optical disk, (1) a reduction in a size of a record mark (increase in a recording density) and (2) a conversion of information into combinations of more than the two types of numerical values are considered. However, any one of these items may increase intersymbol interference when the information is reproduced, and thus, a signal quality of the reproduced signal may degrade. To convert original information into combinations of more than the two types of numerical values is referred to as ‘to convert into multi-level information/data’, and also, data thus obtained is referred to as ‘multi-level data’. Further, the number of the levels of the multi-level data is referred to as a multi-level number or the number of multi-levels. Further, a recording method of recording information after converting the information into multi-level information is referred to as a multi-level recording method.
Various apparatuses for carrying out waveform equalization on a reproduced signal have been proposed (for example, see Japanese Laid-open Patent Applications Nos. 2003-85764 and 2002-319138). According to Japanese Laid-open Patent Application No. 2003-85764, a waveform equalizer is proposed for a binarized recording method. There, waveform equalizing characteristics are switched for a case of applying it to a reproduced signal of a mark and for a case of applying it to a reproduced signal of a space. In this method, the waveform equalizing characteristics are fixed for a mark reproduced signal without regard to a length of each mark.
According to Japanese Laid-open Patent Application No. 2002-319138, an information recording and reproducing apparatus is proposed for the multi-level recording method. There, waveform equalizing characteristics in a demodulating circuit are corrected based on a reproduced signal of calibration information recorded in an information recording medium.
Also in the binarized recording method, the intersymbol interference comes to have a non-linearity as a size of the mark is further reduced in future. Therefore, in the waveform equalizer according to Japanese Laid-open Patent Application No. 2003-85764, since the waveform equalizing characteristics are fixed without regard to the length of each mark, an influence of intersymbol interference may still remain. As a result, it may become difficult to finally obtain a reproduced signal having a necessary signal quality.
Further, according to the multi-level recording method, when mark separation is further narrowed, intersymbol interference comes to have a non-linearity, and as a result, an influence of the intersymbol interference may still remain even in the information recording and reproducing apparatus according to Japanese Laid-open Patent Application No. 2002-319138. As a result, it may become difficult to finally obtain a reproduced signal having a necessary signal quality.